A metal honeycomb for a catalyst typically consists of thin metal sheets, which form a multiplicity of channels through which gas may penetrate the honeycomb. Typically the honeycomb consists of two metal sheets, the one of which is corrugated and the other is substantially smoother so that a multiplicity of channels is generated by combining the sheets in alternating layers. A ceramic support is added onto the surface of the honeycomb, increasing the geometrical area of the catalyst, and including compounds which intensify the activity; the said compounds may, for example, store gaseous compounds. Additionally, the support acts as a base for precious metals which are the actual catalytically active components. The ceramic support may be added onto the surface of the metal sheets before the honeycomb is manufactured, the method being then called `open coating`; or the support may be added after rolling, composing or bending of the honeycomb, when the expression `honeycomb coating` is used.
In order to operate, the catalyst has to reach an appropriate operation temperature (ignition temperature) which may be, for example, over 250.degree. C., depending on the precious metal loading of the catalyst and the type of engine. The tightening of emission limits has resulted in the need to reach the ignition temperature as quickly as possible. Previously, heating a smaller electric catalyst installed in front of the actual catalyst with electric power was considered to be one solution for fast ignition. Thus it was possible to maintain the catalyst's traditional position under the car chassis. However, this solution has been almost totally abandoned, due to costs and problems related with technology. It has become an established practice to solve the demand for fast ignition by installing a catalyst or a smaller precatalyst directly in connection with the exhaust manifold into a so-called `close coupled` position which, however, makes great demands on the durability of the catalyst, because the thermal and mechanical stresses applied to the catalyst are considerable.
In the `close coupled` position, large acceleration forces are applied to the catalyst on frequencies typically of 50-400 Hz. At worst these acceleration values transmitted from the engine body to the catalyst via the exhaust manifold are several dozens of times the earth gravity acceleration. Further, pulsive impulses of the exhaust gas flow affect the catalyst honeycomb and especially its frontal surface said impulses causing vibrations of the frontal surface of the honeycomb at the combustion frequency of the engine. In this connection, the expression high-frequency fatigue is used of these phenomena.
In addition to the high-frequency fatigue, a considerable cycling of thermo-thermal forces is directed to the catalyst honeycomb which are worst in the `close coupled` position. When starting a car or when increasing fast the load of the motor, the exhaust gas temperature rises, and also the concentrations of the components to be oxidized in the catalyst increase, leading to the heating of the catalyst and to a drastic rise in temperature, at first especially in the central parts of the honeycomb. Thermal expansion of the metal honeycomb due to thermal gradient against the colder jacket surrounding the honeycomb leads to compression stress in the honeycomb; this means that, in high stresses in temperatures of about 900-1000.degree. C. the honeycomb, which is typically manufactured of ferrite steel, is unavoidably deformed to some extent, depending, for example on the size of the said thermal gradient, which again is affected by, for example, external or internal thermal insulation used in the jacket, and flow distribution. When the engine load is reduced or the engine is turned off, the engine temperature falls, and the honeycomb and the jacket contract. The honeycomb then tends to take the new, smaller volume it adopted in the high temperature, and a tensile stress is generated between the honeycomb and the jacket, which at worst leads to the tearing away of the honeycomb from the casing. In addition to the radial direction, temperature gradients are also present in the honeycomb in the flow direction, i.e. in the axial direction, causing extra thermal stresses. The forces generated by thermal cycling are called thermal fatigue.